The present disclosure relates to hydraulic braking systems and more particularly to a brake booster used with a braking master cylinder.
Generally, hydraulic braking systems include an element, such as a drum, associated with a vehicle wheel, brake shoes, a backing plate, and a hydraulic cylinder. The drum includes a braking surface against which the brake shoes bear when the braking system is activated. The backing plate does not rotate with the wheel or drum, but instead supports the hydraulic cylinder, the brake shoes, and the other braking components.
The hydraulic cylinder, or master cylinder, incorporates one or more pistons which force the brake shoes against the braking surface of the drum to stop or to limit the rotation of the drum. To activate the braking system an actuator, such as a foot pedal, is moved, thereby causing an increased hydraulic pressure in the master hydraulic cylinder. This increased pressure forces the piston(s) to extend from the master cylinder to drive the brake shoes.
In certain heavier vehicles, the hydraulic braking system may include a booster that is coupled to the master cylinder to magnify the hydraulic pressure applied to the brake shoes. A typical booster scavenges pressure from a source outside the braking system, such as from the power steering hydraulics of the vehicle, and uses that hydraulic pressure to drive a power piston within the booster. The power piston applies a force to the master cylinder piston(s) to thereby increase the braking force available through the braking system.
By way of example, FIG. 1 depicts a brake booster 10 coupled to a master cylinder 12 of a hydraulic braking system. The booster housing 11 is mounted to the housing 13 of the master cylinder with the power piston 16 of the booster slidably disposed within a bore 11a and axially aligned with the primary piston 18 of the master cylinder. The booster 10 includes a throttle 19 connected to the braking actuator, such as the foot pedal, that is operable to activate the power piston 16. The booster 10 includes a return spring 24 that is operable to return the power piston 16 to the neutral position shown in FIG. 1 when the braking actuator has been released. One end of the spring 24 bears against the head 16a of the piston disposed at one end, or piston end, of the bore 11a. The opposite end of the return spring must bear against a reaction surface, which includes the master cylinder housing 13 when the booster 10 is coupled to the master cylinder 12 at the opposite end, or mating end 11b, of the bore 11a. 
However, prior to mounting the booster 10 it is necessary to retain the return spring 24 within inner bore 11a of the booster housing as the booster 10 is being assembled. Consequently, an end cap 20 has been included in prior booster devices that provides the reaction surface to retain the spring. The end cap 20 is supported on the power piston 16 at the mating end 11b of the bore 11a, as shown in the enlarged view of FIG. 2. The end cap 20 includes an inner hub 21 that may be sized for a close running fit with the outer surface of the piston. Alternatively, a seal 22 may be engaged over the inner hub 21 to provide a tight fit between the hub and the piston; however, the tight fit provided by the seal 22 is not sufficient to resist the force of the return spring 24. Consequently, a retaining ring 25 is provided that is retained in a circumferential groove 26 formed near the end of the power piston. The retaining ring 25 extends radially outward to contact a mating face 23 of the end cap 20. The retaining ring may be in the form of a snap ring or a lock washer that snaps into the groove 26 in the power piston.
The retaining ring 25, mounted to the end of the power piston 16, thus traps the return spring 24 within the inner bore 11a of the brake booster 10. One end of the spring bears against the head 16a of the piston while the other end bears against the end cap 20 held by the retaining ring 25. However, as shown in the detail view of FIG. 2, when the brake booster 10 is mounted to the master cylinder 12, the inner mating surface 14 of the master cylinder housing 13 provides the reaction surface against which the end cap 20 bears under the force of the return spring 24. The housings 11 and 13 are typically toleranced so that when the housings are coupled together the mating face 14 of the master cylinder housing 13 essentially pushes the end cap 20 away from the retainer ring 25. In other words, when the booster is assembled with the master cylinder the retainer ring 25 becomes superfluous since it can no longer contact the end cap 20.
In this prior brake booster, the interface between the end cap 20 and the booster housing 11 is configured to facilitate insertion of the end cap into the inner bore 11a. Thus, as shown in the detailed view of FIG. 3, the end cap includes an outer circumferential rim 30 that is sized for a close running fit with the inner bore 11a. In certain boosters, a seal 34 may be engaged over the outer rim 30 to provide a tight fit between the rim and the housing. As with the seal 22 described above, the tight fit provided by the seal 34 is not sufficient to resist the force of the return spring 24. The end cap 20 includes a spring retainer surface 32 that provides the reaction surface for the spring 22 and helps retain the spring in axial alignment with the power piston 16.
Also shown in FIG. 3 is the relationship between the master cylinder housing 13 and the end cap 20. In particular, the outer mating surface 15 of the master cylinder housing 13 contacts the booster housing 11. The inner mating surface 14 is offset in relation to the outer mating surface 15 to account for the offset of the end cap mating face 23 relative to the end of the outer rim 30. The tolerance stack between the master cylinder housing mating surfaces 14 and 15 and the booster housing 11 is such that the master cylinder housing 13 slightly compresses the end cap 20 against the return spring 24 when the two housings are coupled. This arrangement renders the retaining ring 25 no longer necessary, as explained above.